The present invention concerns a back-up power system for a real-time clock. More particularly, the invention concerns a back-up battery system for providing secondary power to the real-time clock of an on-board device controller, such as an engine control module for a vehicle.
The microprocessor has found widespread application in the automotive industry. In one application, an engine control module provides software-based control of the operation of a vehicle engine. In addition, the engine control module (ECM) continuously monitors engine performance. For instance, a plurality of condition sensors are disposed at various components of the engine. These sensors are periodically polled by the ECM and the resulting output signals are stored as data in ECM memory for subsequent analysis. This data can be downloaded by an engine technician using an external tool.
In order to provide more comprehensive diagnostics of engine activity and performance, many ECMs include a capability for logging performance data as a function of actual time. For example, some ECMs log the times at which an engine is started, idled, restarted, or stopped. An addition, time and date stamps can be applied to engine fault data. Still other ECM's keep track of maintenance hours, namely the elapsed time between oil, coolant and filter changes.
In order to provide this actual time information, the typical ECM includes an on-board real-time clock. The real-time clock runs continuously and maintains accurate date and time information. In a typical engine controller installation, the real-time clock is provided as a hardware peripheral, such as the Motorola MC68HC68TI. This peripheral chip outputs real time as a binary coded decimal signal in the format of year/month/day/hour/minute/second/millisecond. This BCD value is then used by the on-board ECM to compute other time-related values.
The real-time clock (RTC) is continuously powered even when the vehicle engine has been shutdown or the keyswitch has been turned to the OFF position. In a typical application, the ECM includes a power supply transformer that is directly connected to the on-board vehicle battery. This transformer can provide power to various essential components of the vehicle electrical and control system, such as elements of the ECM, certain sensors, and the real-time clock.
In the event that power from the primary on-board vehicle battery is lost, most RTC chips can recognize the condition and set a flag. This flag can be subsequently read by the ECM, which itself sets a flag indicating that the real-time clock information is no longer accurate. Some engine control modules will continue to calculate time-related information based upon the output of the RTC when normal power is restored. When data is subsequently downloaded from the ECM, the technician can then evaluate the time and date stamped information in light of the power loss flag set by the ECM. This tagged information can be subsequently reconstructed by the technician, although the reconstruction techniques are subject to certain inaccuracies. In one typical ECM, the latest real-time value is stored in non-volatile memory when the ECM is shut down. However, when power is abruptly lost, many ECM's do not have a mechanism for recording the last known real time.
There is therefore a need for a back-up power supply that is operable in the event of a failure of the primary power source. Preferably, the back-up power supply is capable of replacement or regeneration so that the back-up power is always available on demand.